I06 Mr Stewart an the Causes of Obstruction in 



diminish its diameter (even to the verge of creating a vacuum 

 between it and the sides of the pipe), and to weaken its corpus- 

 cular attraction, which is now sustaining almost the whole action 

 of the gravitating force ; just as, when it issues from the mouth 

 of the pipe, its velocity progressively increases, its diameter con- 

 tracts, and its corpuscular attraction diminishes as it falls to- 

 wards the earth, until, if the height of its fall be great, its cohe- 

 sion is at length entirely lost, and it is divided and scattered into 

 spray. That this diminution of the corpuscular attraction really 

 has the effect of decreasing the capacity of the fluid for retaining 

 the combined air, appears from the fact, that, in the circumstan- 

 ces described, the air is found to be gradually disengaged, and 

 to lodge in those parts of the pipe which have the smallest de- 

 gree of inclination ; in which situation its natural buoyancy has 

 less power to force its way against the opposing action of the 

 current, obstructing by its presence, or entirely stopping, the 

 flow of the water through the pipe. 



This tendency of the disengaged air to lodge in the horizontal 

 parts of water pipes, and in the top of draining syphons, often 

 occasions much trouble and inconvenience ; and the ordinary 

 means of expelling it even from water-pipes by air-cocks and 

 the forcing syringe, appeared to the writer of this paper so in- 

 convenient and ineffective, that he conceived the idea of the ap- 

 plication to his own water-pipes of a double air-vessel for extract- 

 ing the imprisoned air. This attention being afterwards drawn 

 to the draining-syphon, in which the difficulty of expelling the 

 confined air is still greater, and the means of effecting it still 

 more imperfect, this air-vessel appeared to him no less applicable 

 to the syphon than to water-pipes ; and, after some experiments 

 with a working model, it assumed the form which he is about to 

 describe, and which he is persuaded will prove effectual in prac- 

 tice for both purposes. 



